As a process for producing a flat glass, a tin float process employing molten tin, a pull method, a down draw process, a fusion process and the like have been known.
However, in the above tin float process, since a glass ribbon is floated on a medium having a higher thermal conductivity than glass (on a float bath of molten tin) for forming, the transfer of heat to the medium is significant, and the influence of the temperature of the medium is significant, and accordingly the temperature control of the medium is very important. Further, gradual cooling is required under strict control by making the temperature of the medium close to the temperature of the glass so that the difference in the temperature between the surface and the inside of the glass ribbon during cooling is small. Accordingly, the cooling has to be carried out slowly, an adequately long float bath is required, and accordingly the forming time tends to be long. Further, in view of quality also, various drawbacks derived from tin are likely to occur. Further, exhaustion of tin resource is concerned.
On the other hand, in the pull method, the down draw process and the fusion process, due to forming in a vertical direction, it tends to be difficult to control the force resulting from the gravity to the glass ribbon, whereby it tends to be difficult to control the thickness of the glass, and the temperature control of the medium to decrease the difficulty becomes complicated.
Under these circumstances, the present applicant has proposed a technique relating to a process for forming a flat glass by forming a glass ribbon in a molten state into a plate shape via a thin layer of a steam film by using a base containing a steam film forming agent (hereinafter referred to as “steam float process”) (JP-A-9-295819). Further, the present applicant have filed also an application regarding peripheral technology relating to the process for producing a flat glass. According to this process for producing a flat glass, such effects as resource saving, energy saving, high quality of the flat glass, decrease in equipment and operation cost, easiness of job change and possibility of variety of applications to small scale production to large scale production, can be obtained.
An apparatus for producing a flat glass by this steam float process is as shown in FIG. 5. A plurality of rectangular supports 12, 12 . . . formed by a material capable of containing a liquid therein or formed into a structure capable of forming a liquid therein are arranged and fixed on the surface of an endless belt 20A of a belt conveyer 20 with grooves 12B with a certain distance in a longitudinal direction, and the supports 12 make a circuit by the movement of the endless belt 20A. The supply of a steam film forming agent such as water to the supports 12 is carried out when the supports pass through a supply apparatus 29 provided on the lower side of the circuit. When a molten glass G having e.g. the viscosity adjusted by a glass melting furnace 14 is supplied as a high temperature glass ribbon 13 from a slit exit 14A on the supports 12 making a circuit, the steam film forming agent is instantaneously vaporized from the support 12 to form a thin layer 18 of a steam film between the supports 12 and the glass ribbon 13. Further, the circling moving speed of the supports 12 and the moving speed of the glass ribbon 13 which moves on the supports 12 are set different from each other, so that the supports 12 and the glass ribbon 13 are made to slide on each other. Accordingly, the glass ribbon 13 is formed into a thin flat glass on the thin layer 18 of the steam film. In FIG. 5, the numerical references 20C and 20D represent a driving roll and a driven roll between which the endless belt 20A is stretched, the numerical reference 21 represents a guide plate which guides the upper moving path of the endless belt 20A, the numerical reference 23 represents carriage driving rollers which carry the glass ribbon, and the numerical references 26 and 28 represent the circuit direction of the supports, which may be either in a clockwise direction or in an anticlockwise direction.
However, in the case of a transfer type in which the supports 12 make a circuit, the thickness of the steam film or the pressure of the steam film tends to be non-uniform in the glass ribbon 13 moving direction, whereby a thickness distribution or a steam film pressure distribution of the thin layer 18 of the steam film is likely to generate. This generation of the thickness distribution or the steam film pressure distribution of the steam film may deteriorate the surface smoothness of the formed flat glass in the moving direction in some cases.
Further, in the case of the transfer type in which the supports 12 make a circuit, the landing position of the glass ribbon 13 supplied from the glass melting furnace 14 to the supports 12 and the position of grooves 12B between the supports 12 periodically agree with each other, and accordingly the glass ribbon 13 instantaneously flows down into the grooves 12B when they agree with each other, which may cause stripes on the formed flat glass.
Accordingly, in order to improve quality of a flat glass by the steam float process, the above problems have to be resolved.
The present invention has been made under these circumstances, and it is an object of the present invention to provide a process and an apparatus for producing a flat glass, which can uniformalize the surface smoothness of the formed flat glass in the moving direction and prevents formation of stripes on the flat glass.